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Mac OS X

Text version: Use the GUI version, and do
a “Unison -> Install Command line tool” GUI Intel

2.48.6
(2015.02.14, stable. Require a version of OS X with the
notification center. Compiled with Ocaml 4.02.) SHA1:
211f283fa3cf974a18a39a697dc5b9d00ab77a5e.

2.48.3
(2015.06.17, stable. Require a version of OS X with the
notification center. Compiled with Ocaml 4.01; use this version if
you need to connect to unison servers compiled with Ocaml
4.01.) SHA1: b77d588bdee11a50c0ed90acb7e3558499f9cfce.

Windows

Thanks to Karl M, here are native MSVC Windows version
(GTK+, at least version 2.12, and text
versions in each zip file):
2.27.157 (2009.06.30, stable), 2.32.52 (2009.06.30, stable), 2.40.102 (2013.01.22, stable).
Karl is using Pidgin GTK+ 2.14.7a for the 2.32 version. A
runtime environment may be downloaded from the Pidgin
project. If a gtk dll cannot be found (such as
libgtk-win32-2-0.0.dll), it may be useful to add
C:\Program
Files\Common Files\GTK\2.0\bin to the PATH
variable in the system settings.
You may also need to install the VC redistributable if it’s not
present on your system.

Bernhard Fisseni had the following tip regarding these binaries.The GTK from
Pidgin gives me errors about undefined symbols in libglib2. What works
without further ado, however, is installing graphviz
(http://www.graphviz.org), which also seems to bring its own GTK, and
adds itself to the PATH.
In the previous quote, the undefined error message is the following:

The procedure entry point g_malloc_n could not be located in the dynamic
link library libglib-2.0.0.dll

An alternative to the installation of the whole graphviz application is the gtk-win project,
which also includes the appropriate directory to the path (Thanks to Cameron
Davidson for the tip).
Recent binaries may also be found on Jérôme Vouillon’s page.